1. Field of the Invention
The present invention relates generally to packaging of semiconductor dice and, more specifically, to packaging of flip-chip configured semiconductor dice employing an interposer substrate having recesses in one or both sides thereof for receiving discrete conductive elements projecting from the semiconductor dice.
2. State of the Art
Chip-On-Board (“COB”) or Board-On-Chip (“BOC”) technology is used to attach a semiconductor die directly to a carrier substrate such as a printed circuit board (“PCB”), or an interposer may be employed and attachment may be effected using flip-chip attachment, wire bonding, or tape automated bonding (“TAB”).
Flip-chip attachment generally includes electrically and mechanically attaching a semiconductor die by its active surface to an interposer or other carrier substrate using a pattern of discrete conductive elements therebetween. The discrete conductive elements are generally disposed on the active surface of the die during fabrication thereof, but may instead be disposed on the carrier substrate. The discrete conductive elements may comprise minute conductive bumps, balls or columns of various configurations. Each discrete conductive element is placed corresponding to mutually aligned locations of bond pads (or other I/O locations) on the semiconductor die and terminals on the carrier substrate when the two components are superimposed. The semiconductor die is thus electrically and mechanically connected to the carrier substrate by, for example, reflowing conductive bumps of solder or curing conductive or conductor-filled epoxy bumps. A dielectric underfill may then be disposed between the die and the carrier substrate for environmental protection and to enhance the mechanical attachment of the die to the carrier substrate.
Wire bonding and TAB attachment techniques generally begin with attaching a semiconductor die by its back side to the surface of a carrier substrate with an appropriate adhesive, such as an epoxy or silver solder. In wire bonding, a plurality of fine wires is discretely attached to bond pads on the semiconductor die and then extended and bonded to corresponding terminal pads on the carrier substrate. A dielectric encapsulant such as a silicone or epoxy may then be applied to protect the fine wires and bond sites. In TAB, ends of metal traces carried on a flexible insulating tape such as a polyimide are attached, as by thermocompression bonding, directly to the bond pads on the semiconductor die and corresponding terminal pads on the carrier substrate.
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. As new generations of integrated circuit products are released, the number of components used to fabricate them tends to decrease due to advances in technology even though the functionality of the products increase. For example, on the average, there is approximately a ten percent decrease in components for every product generation over the previous generation having equivalent functionality.
Recent trends in packaging are moving with increasing rapidity toward flip-chip attachment due to improved electrical performance and greater packaging density. However, flip-chip attachment is not without problems, such as the high cost for a third metal reroute of bond pads from the middle or periphery of a die to a two-dimensional array which, in turn, may result in overlong and unequal length electrical paths. In addition, many conventional flip-chip techniques exhibit a lack of consistent reliability of the interconnections between the chip and the interposer or other carrier substrate as a result of the increased miniaturization as well as difficulties in mutual alignment of the die and carrier substrate to effect such interconnections. Effective rerouting of bond pads may also be limited by die size. Another hindrance to flip-chip packaging has been difficulty in electrically testing completed flip-chip semiconductor device assemblies using existing test probe equipment. Thus, even if a semiconductor die in the assembly is a so-called “known good die,” the assembly itself may exhibit defects that are not easily detected and that may, even if detected, be at a stage in the fabrication process subsequent to encapsulation, rendering rework of the assembly difficult if not impossible.
Further, flip-chip packages for a bumped semiconductor die employing an interposer may be undesirably thick due to the combined height of the die and interposer. This is due to the use in conventional packaging techniques of relatively costly interposers comprising dual conductive layers having a dielectric member sandwiched therebetween, the bumped semiconductor die resting on and connected to traces of the conductive layer on one side of the interposer and electrically connected to traces of the conductive layer on the opposing side, conductive vias extending therebetween. Finally, underfilling a flip-chip-attached semiconductor die to a carrier substrate with dielectric filler material can be a lengthy and often unreliable process, and the presence of the underfill makes reworking of defective assemblies difficult if not impossible.
Other difficulties with conventional packages include an inability to accommodate die size reductions, or “shrinks,” as a given design progresses through several generations without developing new interposer designs and tooling. As more functionality is included in dice, necessitating a greater number of inputs and outputs (I/Os), decreased spacing or pitch between the I/Os places severe limitations on the use of conventional interposers. In addition, with conventional packages, a die is not tested until package assembly is complete, resulting in excess cost since a defective die or die and interposer assembly is not detected until the package is finished.
For example, U.S. Pat. No. 5,710,071 to Beddingfield et al. discloses a fairly typical flip-chip attachment of a semiconductor die to a substrate and a method of underfilling a gap between the semiconductor die and substrate. In particular, the semiconductor die is attached face down to the substrate, wherein conductive bumps on the die are directly bonded to bond pads on the upper surface of the substrate, which provides the gap between the die and substrate. The underfill material flows through the gap between the semiconductor die and the substrate via capillary action toward an aperture in the substrate, thereby expelling air in the gap through the aperture in the substrate in an effort to minimize voids in the underfill material. However, such an underfilling method still is unnecessarily time consuming due to having to underfill the entire semiconductor die. Further, the flip-chip attachment technique disclosed in U.S. Pat. No. 5,710,071 exhibits difficulties in aligning the conductive bumps with the bond pads on the substrate and requires the expense of having a third metal reroute in the substrate.
Therefore, it would be advantageous to improve the reliability of interconnections between a chip and a carrier substrate such as an interposer by achieving accurate alignment of the interconnections, an improved underfill process, and the elimination of the necessity for a third metal reroute, while reducing total assembly height in combination with the ability to employ commercially available, widely practiced semiconductor device fabrication techniques and materials as well as existing test equipment.